Chlorine is shipped as a pressurized liquid in tank cars. Customers use the chlorine by forcing it out of the tank cars with air pressure. When the tank cars are returned to the chlorine manufacturer, they are filled with air that contains some of the chlorine that was not removed. Gas mixtures that contain small amounts of chlorine and relatively harmless other gases also come from barges, plant vents, and other sources, and are known as "snift gases." Because snift gases contain chlorine they cannot be vented to the atmosphere.
At the present time, snift gases are passed through an absorber column where the gas contacts liquid carbon tetrachloride. The carbon tetrachloride absorbs the chlorine and the chlorine-free air is vented to the atmosphere. The carbon tetrachloride containing the absorbed chlorine is heated to vaporize the chlorine. The vaporized chlorine is liquified and recovered and the carbon tetrachloride, now free of chlorine, is recycled to the absorber column.
The problem with this chlorine recovery process is that small amounts of carbon tetrachloride vaporize and are vented to the atmosphere with the air. Because U.S. Government personnel have determined that carbon tetrachloride is an ozone depleter, the use of carbon tetrachloride as a chlorine absorber will no longer be permitted. Efforts are therefore underway to identify liquids that are good chlorine absorbers, but which are relatively non-volatile and are not ozone depleters.